1. Field of the Invention
The present invention relates to a method and a radio communication system for synchronizing subscriber stations, particularly to the synchronization within a mobile radio system which includes broadband channels and TDD and CDMA subscriber separation.
2. Description of the Prior Art
In radio communication systems, messages (for example, voice, video information or other data) are transmitted via an air interface with the aid of electromagnetic waves. The air interface relates to a connection between a base station and subscriber stations, where the subscriber stations can be mobile stations or stationary radio stations. The electromagnetic waves are radiated with carrier frequencies which are in the frequency band provided for the respective system. For future radio communication systems, for example the Universal Mobile Telecommunication System (UMTS) or other third-generation systems, frequencies within the frequency band of approx. 2000 MHz are provided.
For the third mobile radio generation, two modes are provided; one mode designating an FDD (Frequency Division Duplex) mode, see ETSI STC SMG2 UMTS-L1, Tdoc SMG2 UMTS-L1 221/98, of 25.8.1998, and the other mode designating a TDD (Time Division Duplex) mode, see DE 198 27 700. The operating modes are used in different frequency bands and both use time slots.
In ETSI STC SMG2 UMTS-L1, Tdoc SMG2 UMTS-L1 221/98, of 25.8.1998, a synchronization method which uses synchronization sequences transmitted in every time slot is described in chapters 2.3.3.2.3 and 6.3 for the FDD mode. This makes it possible to synchronize the subscriber stations at the beginning of the time slot. The sequence of transmissions of a second synchronization sequence signals which spread-spectrum code is used by the base station. Furthermore, the beginning of the frame also can be derived therefrom.
However, this synchronization method leads to a scenario as shown in FIG. 1 with a synchronous operation of the base stations. The synchronization sequences of base stations of two radio cells Z1, Z3 are superimposed at the receiving subscriber station since the transmissions of the synchronization sequences of all base stations are referred to at the beginning of a frame; see FIG. 10 in ETSI STC SMG2 UMTS-L1, Tdoc SMG2 UMTS-L1 221/98 of 25.8.1998. The superimposition impairs proper synchronization.
It is, therefore, an object of the present invention to specify a method and a radio communication system via which the synchronization of the subscriber stations is possible without errors even with at least partially synchronized base stations.